The voice bandwidth communications channel, roughly 300 to 3000 Hz is a common industry standard. Accordingly, a variety of modems are offered to encode digital data onto a voice channel at one point and to decode that digital data elsewhere at rates in bits per second of 600, 1200, 2000, 2400, 3600, 7200, or faster. In general, modems for higher bit rates are more complex and costly; they introduce and interpret more subtle channel signal variations; and, they require communication channels having more stringent amplitude and relative delay characteristics. In many situations the quantity or nature of data to be sent will warrant the use of more sophisticated modems and careful attention to channel transmission parameters.
In situations involving only a few locations the cost of the transmission channel can be justified while in other situations it cannot. This is particularly true in the case of offshore shallow water platforms equipped with crude oil treating facilities. On the average, approximately twentyfive nearby producing wells at randomly spaced individual well jackets are associated with a treating facility platform. A separate flow line from each well buried beneath the ocean floor conducts produced oil, water and gas into the production header manifold of treating facility. Normally the wells use injected gas for artificial lift and are subject to liquid flow rates that fluctuate widely with time. Devices are available for indicating liquid flow rate in each incoming flow line at the treating facility. While the flow devices are available at low cost, reliable communications capable of high speed digital transmissions are not. A communication system would allow frequent real time sampling of instantaneous flow rates from which the computer could determine the smoothed average liquid flow rate of each well.
In a typical shallow water field all platforms lie within an area approximately circumscribed by a semi-circle of ten miles radius about a central facility and pipeline terminal located on the beach. A microwave terminal station at the central facility is part of a system extending all the way back to the computer location 400 miles away. Thus, with suitable communications readily available between central facility and the computer center, only the short-distance communications between central facility and the twenty platforms remains.
If only a few platforms were involved an extension of the microwave system might afford a ready solution. In the above example the cost of twenty microwave stations and the potential for self-interference of that much microwave in so small an area eliminates microwave from consideration. Submarine cable was considered and rejected on the basis of exorbitant maintenance cost and limited reliability expected as a result of construction activities, fishing and boats anchoring within the field. Attempts have been made to use separate radio frequency channel assignments for each platform. A single transmitter was installed on the central platform for working into a receiver on each satellite location with separate receivers being required at the central location to handle individual satellite transmitted frequencies. These systems, operating with transmitters continuously keyed, experience some difficulty handling 600 bits per second data. On the scale of twenty platforms a separate frequency for each location probably could not be justified to the satisfaction of the Federal Communications Commission.